Using /126 for IPv6 router links

Sun Jan 24 23:41:18 UTC 2010

On Jan 24, 2010, at 6:26 PM, Valdis.Kletnieks at vt.edu wrote:

> On Sun, 24 Jan 2010 17:01:21 EST, Steven Bellovin said:
> 
>> Actually, Scott Bradner and I share most of the credit (or blame) for
>> the change from 64 bits to 128.
>> 
>> During the days of the IPng directorate, quite a number of different
>> alternatives were considered.  At one point, there was a compromise
>> proposal known as the "Big 10" design, because it was propounded at the
>> Big Ten Conference Center near O'Hare.  One feature of it was addresses
>> of length 64, 128, 192, or 256 bits, determined by the high-order two
>> bits.  That deal fell apart for reasons I no longer remember;
> 
> I don't remember the details of Big 10, but I do remember the general objection
> to variable-length addresses (cf. some of the OSI-influenced schemes) was the
> perceived difficulty of building an ASIC to do hardware handling of the
> address fields at line rate.  Or was Big 10 itself the compromise to avoid
> dealing with variable-length NSAP-style addresses ("What do you mean, the
> address can be between 7 and 23 bytes long, depending on bits in bytes 3, 12,
> and 17?" :)

Precisely.  The two bits could feed into a simple decoder that would activate one of four address handlers; depending on your design, they could all run in parallel, with only the output of one of them used.  There were only four choices, all a multiple of 8 bytes.

But my goal is not to revisit the design issues, but rather to clarify the historical record.

		--Steve Bellovin, http://www.cs.columbia.edu/~smb